C.dbd.C in lipids.
The annual production of vegetable oils is about 90.million tons (Mielke 1992), of which about 20% are hardened (hydrogenated). Furthermore, about 2 million tons of marine oils are hydrogenated yearly. The production is spread over the whole industrialized world. Through the hydrogenation, hydrogen is added to the double bonds of the unsaturated fatty acids. The largest part of the oils is only partly hydrogenated. The desired conditions of melting and the desired consistency of the fats are thereby obtained, which are of importance for the production of margarine and shortening. The tendency to oxidation is reduced by the hydrogenation, and the stability of the fats is increased at the same time (Swern 1982).
In the future, the lipids may be modified by methods belonging to bio technology, especially gene technology, but hydrogenation will certainly remain.
A problem with the hydrogenation processes of today is, that new fatty acids are produced which to a great extent do not exist in the nature. They are often called trans fatty acids, but the double bonds change position as well as form (cis-trans) during the hydrogenation (Allen 1956, Allen 1986).
As a rule, trans fatty acids are desired from a technical and functional point of view (Swern 1982), but regarding health, their role is becoming more and more questionable (Wahle & James 1993).
A typical state of the art reactor for hydrogenation is a large tank (5 to 20 m.sup.3) filled with oil and hydrogen gas plus a catalyst in the form of fine particles (nickel in powdery form). The reaction is carried out at a low pressure, just above atmospheric (0.5 to 5 bar), and high temperatures (130 to 210.degree. C.). The hydrogen gas is thoroughly mixed into the oil, as this step restricts the reaction rate (Grau et al., 1988).
If the pressure of hydrogen gas is increased from 3 to 50 bar when soya oil is partially hydrogenated (iodine number at the start=135, at the end=70), the content of trans is reduced from 40 to 15%. The position isomerization is also reduced to a corresponding level (Hsu et al., 1989). These results are of no commercial interest, as these conditions enforce a replacement of the low pressure autoclaves by high pressure autoclaves.
According to the "half hydrogenation" theory, the concentration of activated H-atoms on the catalyst surface determines the number of double bonds being hydrogenated and deactivated without being hydrogenated respectively. A lack of activated H-atoms causes a trans- and position-isomerization (Allen 1956, Allen 1986). A lack of activated
H-atoms can be the consequence of low solubility of H.sub.2 in the oil, or of a bad catalyst (poisoned or inadequately produced). Thus, the "half hydrogenation" theory corresponds very well to the empirical results (Allen 1956; Allen 1986; Hsu et al., 1989).
It is possible to deodorize and hydrogenate an oil in the presence of CO.sub.2 and hydrogen (Zosel 1976). Zosel describes in detail how to use CO.sub.2 in order to deodorize the oil. However, it must be emphasized that Zosel does not give any hint, that CO.sub.2 should have an influence on the hydrogenation process. Furthermore, Zosel does not touch on the cis/trans problem.
In the experiments of Zosel, the catalyst is surrounded by a liquid phase during the entire process. Zosel does not disclose the composition, but in the light of the other data, we estimate that the liquid phase consists of oil (about 95%), CO.sub.2 (about 5%) and hydrogen (about 0.03%). This phase is far away from a supercritical condition. As a consequence, the velocity of reaction is limited by the concentration of hydrogen on the catalyst surface. The same applies to all traditional hydrogenation reactions where the catalyst is in the liquid phase as well. The velocity of hydrogenation in the experiments of Zosel is about 100 kg/m.sup.3 h, i.e. somewhat lower than in traditional hydrogenizing reactors.
FATTY ALCOHOLS.
Fatty alcohols and their derivatives are used in shampoo, detergent compositions and cosmetic preparations etc. The annual production is about 1 million tons. About 60% is based on petrochemicals, and about 40% is derived from natural fats and oils. The raw material for short chain fatty alcohols, C.sub.12 -C.sub.14, is coco-nut oil and palm kern oil, whereas C.sub.16 -C.sub.18 comes from tallow, palm oil or palm stearin (Kreutzer 1984, Ong et al., 1989).
It is theoretically possible to hydrogenate triglycerides, fatty acids and methylesters to fatty alcohols. A direct hydrogenation of triglycerides has not been developed commercially, because the glycerol will be hydrogenated as well and thus lost. A direct hydrogenation of fatty acids requires corrosion resistant materials and a catalyst resistant to acids (Kreutzer 1984). Lurgi has developed a hydrogenation process (the slurry process), where fatty acids are introduced and are quickly esterified with a fatty alcohol to a wax ester, and then the wax ester is hydrogenated (copper chromite, 285.degree. C., 300 bar)(Buchhold 1983, Voeste Buchhold 1984, Lurgi 1994).
Most plants for the production of natural fatty alcohols are based on methyl esters as raw material. Saturated fatty alcohols are produced at a temperature of about 210.degree. C. and a pressure of 300 bar using copper chromite as catalyst in a fixed bed reactor. Other catalysts as copper carbonate, nickel or copper and chromic oxide will also function (Mahadevan 1978, Monick 1979, Lurgi 1994). Unsaturated fatty alcohols are produced at about 300.degree. C. and 300 bar, normally using zinc chromite as catalyst. There are also other catalysts which selectively hydrogenate the group COOR, leaving the C.dbd.C unimpaired (Klonowski et al., 1970; Kreutzer 1984).
The reaction is limited by the solubility of hydrogen in the liquid (Hoffman Ruthhardt 1993).
Davy Process Technology markets a gas phase process where methyl esters are hydrogenated to fatty alcohols (40 bar, 200 to 250.degree. C., catalyst without chromium) (Hiles 1994).
A lot of work has been done to develop catalysts functioning with less energy (lower temperature, lower pressure). Another object has been to develop methods for a direct hydrogenation of triglycerides to fatty alcohols without a simultaneous hydrogenation of the glycerol (Hoffman Ruthhardt 1993).
HYDROGEN PEROXIDE.
Hydrogen peroxide is used in large quantities for bleaching, cleaning, as a disinfectant and as a raw material in industrial processes etc. Earlier, hydrogen peroxide was derived by an electrolytic process. Now, oxidation of substituted hydroquinone or 2-propanol is most widely used.
There are a lot of patents concerned with direct synthesis of hydrogen peroxide from oxygen and hydrogen. The reaction medium can be acidic organic solvents or water with organic solvents using noble metals, most often palladium, as catalyst (EP-B-0049806; EP-B-0117306; U.S. Pat. No. 4,336,239; EP-B-0049809).
It is preferred that the reaction medium is free from organic constituents because of problems with purification. Several patents use acidic water as the reaction medium (pH=1-2) with addition of halides, especially bromide and chloride (&lt;1 mM) and with noble metals or mixtures of noble metals as catalysts (EP-A-0132294; EP-A-0274830; U.S. Pat. No. 4,393,038; DE-B-2655920; DE 4127918 A1).
The velocities of reaction which are disclosed are about 1 kg/m.sup.3 h, and the selectivity (mol hydrogen peroxide/mol hydrogen reacted) is about 75% (DE 4127918 A1).
According to theory, one can expect to obtain- high selectivity with high concentrations of oxygen and hydrogen on the catalyst surface (Olivera et al., 1994).
The object of the present invention is to obtain a very effective process for partial or complete hydrogenation of the substrates mentioned above.
According to the invention, this problem has been solved by mixing the substrate, hydrogen gas and solvent, and by bringing the whole mixture into a super-critical or near-critical state. This substantially homogeneous super-critical or near-critical solution is led over the catalyst, whereby the reaction products formed, i.e. the hydrogenated substrates, will also be a part of the substantially homogeneous supercritical or near-critical solution.
The solvent can be a saturated hydrocarbon or an unsaturated hydrocarbon which on hydrogenation gives a saturated hydrocarbon, e.g. ethane, ethene, propane, propene, butane, butene, or CO.sub.2, dimethyl ether, "freons", N.sub.2 O, N.sub.2, NH.sub.3, or mixtures thereof.
Propane is a suitable solvent for many lipids. CO.sub.2 is a suitable solvent for hydrogen peroxide and water.
The catalyst will be selected according to the reaction which is to be carried out. For a partial or complete hydrogenation of only C.dbd.C bonds, preferably a noble metal or nickel will be selected. For a selective hydrogenation of COOR to C-- OH and HO--R, the catalyst would preferably be a zinc salt, e.g. zinc chromite. For a simultaneous hydrogenation of COOR to C--OH and HO--R and a hydrogenation of C.dbd.C, the preferred catalyst would be copper chromite, another salt of copper or copper free from chrome. For a partial hydrogenation of oxygen to hydrogen peroxide, the preferred catalyst would be a noble metal.
According to the invention, the concentration of hydrogen on the catalyst surface can be controlled to very high levels. The proportion of trans fatty acids in partially hydrogenated fatty products will be much lower according to the invention than by using conventional processes, where the product has been hydrogenated to the same level using the same catalyst. The hydrogenated products will preferably contain less than 10% trans fatty acids.